The present invention relates to materials for capacitors; that is, grain boundary layer dielectric ceramic compositions in which diffusing agents mainly consisting of Bi.sub.2 O.sub.3, Cu.sub.2 O, MnO.sub.2, B.sub.2 O.sub.3, La.sub.2 O.sub.3 and TiO.sub.2 are thermally diffused into the grain boundaries of a semiconductive ceramic mainly consisting of SrTiO.sub.3 or the solid solution of Sr.sub.1-x Ca.sub.x TiO.sub.3 so that the dielectric layers are formed in the grain boundaries.
The grain boundary layer dielectric ceramic compositions which are used for the manufacture of capacitors comprise a semiconductive ceramic composition and a diffusing composition which forms dielectric layers in the grain boundaries of the semiconductive ceramic composition to form a composition having high resistance. The characteristics of capacitors, therefore, are dependent upon the selection of the semiconductive ceramic compositions and the diffusing compositions.
The semiconductive ceramic compositions in accordance with the present invention mainly consisting of SrTiO.sub.3 containing Nb.sub.2 O.sub.5 or a solid solution of Sr.sub.1-x Ca.sub.x TiO.sub.3 and containing Nb.sub.2 O.sub.5. The semiconductive ceramic compositions consisting of SrTiO.sub.3 containing Nb.sub.2 O.sub.5 are well known in the art as disclosed in Japanese Laid-Open Patent Application Ser. Nos. 129896/1974 and 8099/1975, but as the diffusing agents or compositions which are diffused into the grain boundaries of such semiconductive ceramic compositions so as to form the dielectric layers, only Bi.sub.2 O.sub.3 (as disclosed in Japanese Laid-Open Patent Application Ser. No. 129896/1974) and the compositions consisting of Nb.sub.2 O.sub.5 and ZnO (as disclosed in Japanese Laid-Open Patent Application Ser. No. 8099/1975) are known. The diffusing agents or compositions in accordance with the present invention are characterized by consisting of five constituents of Bi.sub.2 O.sub.3, Cu.sub.2 O, MnO.sub.2, B.sub.2 O.sub.3 and La.sub.2 O.sub.3 or consisting of six constituents of the above-described compounds plus TiO.sub.2. Meanwhile, the diffusing agents or compositions containing Bi.sub.2 O.sub.3 and Cu.sub.2 O and MnO.sub.2 are disclosed in Japanese Laid-Open Patent Application Ser. No. 78494/1974, but the combinations consisting of the five or six constituents in accordance with the present invention had not been disclosed. The semiconductive ceramic composition disclosed in the abovedescribed Patent Application consists of SrTiO.sub.3 containing MnO.sub.2 and Nb.sub.2 O.sub.5 or the solid solution thereof and is different from the semiconductive ceramic compositions in accordance with the present invention.
Based on the variations in electrostatic capacitance due to temperature variations, grain boundary layer dielectric ceramic capacitor elements are rated as YR, YB and YA. That is, the capacitor elements which have a temperature coefficient of capacitance of .+-.15% between +85.degree. C. and -25.degree. C. based on a capacitance at 20.degree. C. are rated as YR. Those which have the temperature coefficient of .+-.10% are rated as YB and those which have the temperature coefficient of .+-.5% are rated as YA. Regardless of their ratings, all the capacitor elements preferably have a high dielectric constant .epsilon..sub.a, a high dielectric breakdown voltage V.sub.b and a low dielectric loss tan .delta.. In practice, the YR rated capacitor elements must have dielectric constant .epsilon..sub.a higher than 35,000, a dielectric breakdown voltage V.sub.b higher than 500 V/mm and a dielectric loss tan .delta. of less than 0.01. The YB rated capacitor elements must have a dielectric constant .epsilon..sub.a higher than 20,000, a dielectric breakdown voltage V.sub.b higher than 700 V/mm and a dielectric loss tan .delta. of less than 0.01. The YA rated capacitor elements must have a dielectric constant .epsilon..sub.a higher than 5000, a dielectric breakdown voltage V.sub.b higher than 700 V/mm and a dielectric loss tan .delta. of less than 0.01.